Within the Earth
Karen G. Lloyd (Princeton University Press)

While many shelves are filled with titles about forests, oceans, and deserts, the deep biosphere, an important and intriguing habitat beneath our feet, is often overlooked. Despite a few notable exceptions, literature on ecosystems ranging from the Amazon to Antarctica largely ignores this underground world.

Not anymore. Within the Earth: Discover the Strangest Life on Earth by Karen G. Lloyd serves as a crucial field guide to the underground life we’ve started to uncover. “In fact, we have yet to find the limits of where life ceases to exist,” she states.

The general unawareness of the deep biosphere’s existence reflects our surface-centric worldview. However, Lloyd, a microbial biogeochemist at the University of Tennessee, Knoxville, argues that learning about this life can profoundly change our understanding of existence itself.

She defines the deep biosphere as areas below the seafloor or beneath land where life thrives without sunlight, the primary energy source for most surface organisms. These environments encompass a variety of metabolic processes, from methane production arising from decomposed plants beneath a few centimeters of marsh mud to chemical processes with microbes three kilometers underground.

Discussing these microorganisms, she notes, “It’s as if there are millions of small, low-energy suns scattered throughout the Earth’s crust, each supporting its own underground ecosystem.”

How much life is present? It’s difficult to say. However, Lloyd contends that all estimates are likely underestimated. One claim suggests that marine sediments alone could hold 2.9 x 10²⁹ cells, potentially twice as many as those in continental fractures and pores, presenting astonishing figures.

Advances in genetic sequencing and field research are illuminating these rich ecosystems. Lloyd helps researchers differentiate between microbial species and deduce metabolic functions through DNA alone. This is especially helpful since many deep-dwelling bacteria and archaea have proven impossible to cultivate in surface laboratories.

The fieldwork section discusses how scientists obtain new DNA samples—whether from hydrothermal vents, excavated continental rocks, or dripping water in deep mines. “To understand limits, one sometimes must become the explorer,” Lloyd notes.

Through engaging prose, she recounts her adventures tracking microorganisms from the high deserts of the Andes to the perilous peaks of Costa Rican volcanoes. These stories resemble scenes from action films—caution is essential to avoid slipping on volcanic glass fragments or falling into acid lakes!

Fortunately, this book transcends a mere expedition narrative. It features an extensive and approachable explanation of the chemistry that enables a deep biosphere. Although the equations involved can be complex, Lloyd adeptly guides readers to grasp the chemical frameworks that support these creatures living on the “edge of energy.”

To facilitate this challenging learning curve, she draws parallels between surface ecosystems and our dietary habits to illuminate the underground world. For instance, bacteria that metabolize sulfides are likened to “couch potatoes,” competing with methane-producing “freeloaders” by preserving hydrogen, a universal nutrient—a dramatic ecological narrative reminiscent of the Serengeti. Sulfate-reducing agents in Svalbard’s fjords “have access to a permanently stocked refrigerator.” Engaging and thrilling, her exploration of biogeochemistry is no small feat.

However, the highlight of Lloyd’s book is her assertion that certain forms of deep life may possess a sluggish metabolism, allowing individuals to survive for thousands, or even millions of years. These “eonophiles” (once confirmed to have extraordinarily long life spans) “redefine our preconceived notions about the nature of life,” she asserts. Truly, these lifestyles are alien, and how fortunate we are to uncover more about them right here on Earth!

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